SEDIMENTATION AND DIAGENESIS OF A MIOCENE-PLIOCENE NARROW SHELF MIXED-SYSTEM: CIBAO BASIN, DOMINICAN REPUBLIC

The diverse settings in which carbonate and siliciclastic sediments mix provides a challenge in developing predictive models of lithofacies distribution and sedimentologic heterogeneity. The skeletal carbonate and siliciclastic mud deposits of the Cibao Basin, in the northern Dominican Republic, are examined to better understand the dynamics of shelf sedimentation within a mixed-system and its diagenesis. The study examines the sedimentary components within the Gurabo Formation deposited during the late Miocene to early Pliocene. Deposition occurred during the time of a eustatic transgression (Zanclean Flood).

This outcrop study examines the variations in the sediment grain composition through three shelfal depositional sequences. The advantage to this outcrop is that the majority of the lithofacies can be disaggregated, providing unusual grain-specific information from the Miocene-Pliocene deposit. There is a mixing of carbonate and siliciclastic sediment within each of these sequences. It is noteworthy that in this mud-dominated mixed system there was prominent reef development. In general, the bulk sediment within the transgressive cycles contained a greater abundance of quartz grains (fine sand and silt), and that of the regressive cycles were predominately skeletal carbonate grains.

The diagenetic susceptibilities within each of the depositional cycles differ due to the input of surrounding organic material and the matrix porosity and permeability. The diagenesis within this mixed-system was dynamic in that that there was microbiologically-mediated marine precipitation of calcite cements within the siliciclastic-rich intervals to produce composite cemented grains. The carbonate-rich intervals experienced typical (tropical) meteoric diagenesis with dissolution of aragonite, and precipitation of secondary calcite and dolomite cements.

The results of the sedimentological, petrographical, and geochemical study comprise the necessary data to illustrate the mixing of the components through temporal changes along a compressed shelf (~6 km wide inner shelf to outer shelf) tropical depositional system. This research provides a case study for the sedimentological dynamics along a narrow-shelf carbonate-siliciclastic mixing and the corresponding diagenetic tendencies.